1. Field of Invention
This invention relates generally to a fuel regulating assembly used in carburetors for internal combustion engines, using a moveable member to affect a change in pressure in an internal vent system.
2. Description of Prior Art
Carburetors operate using air pressure differences acting to force a fuel into a bore of the carburetor, and hence to an engine. This fuel flow is through one or more metering orifices. Modern carburetors use multiple systems or circuits to provide the proper fuel/air ratio required for all engine operating parameters. These systems provide a balance between economy and power, enabling maximum power to be delivered by the engine upon demand, but maximum economy whenever possible.
Two basic elements determine the fuel flow in any of these various circuits. The first element is the physical size of the fuel metering orifice, and to a lesser extent, connecting passageways which comprise the particular fuel circuit. The metering orifice is usually sized to be considerably smaller than the other parts of the fuel delivery system, and for the purpose of analyzing fuel delivery, it can be assumed that the metering orifice constitutes the entire fuel delivery system. The second element is the pressure difference existing across the fuel delivery system, or essentially, the pressure existing across the metering orifice.
The pressure difference acting across the fuel metering orifice in its most basic configuration consists of the pressure existing on the fuel in the fuel chamber of the carburetor, less the pressure existing in the carburetor bore where the outlet of the fuel delivery system is located, less the head pressure of the fuel. The pressure existing on the fuel in the fuel chamber is controlled by an average reference pressure established by a vent. If the vent is entirely external to the carburetor and its air induction passage, the venting is called external, and atmospheric pressure is the average reference pressure used for the carburetor. If the vent communicates with a region of the bore or other area of the air induction passage, for instance the air cleaner, this venting is called internal. In this case, the average reference pressure used for the carburetor will be slightly less than atmospheric, depending on the location of the pressure sensing end of the vent. Both types of venting, internal and external, are well known in the art.
The pressure existing in the carburetor bore and other parts of the air induction system, and hence existing at the outlet of the fuel delivery system, is determined by engine operating conditions, the position of a throttle valve, a variable venturi if so equipped, and the shape and cross sectional area of the carburetor bore. As a gas, or in this case air, is moving at a velocity, the pressure measured by a pressure sensing orifice with its surface parallel to this flow will be lower in regions where the velocity is greater. This is called the Bernoulli effect. Therefore, as engine speed is increased and the carburetor throttle is opened, air velocity in the bore increases, and pressures perpendicular to the wall of the bore decrease. Also, the bore is normally shaped so that there is a region having a decreasing cross sectional area, called a converging section. The portion having the smallest cross sectional area is called the throat, and air speed will be highest in this region. The converging section and throat comprise the venturi; most carburetors have a venturi with fixed dimensions, but some carburetors have a venturi with operably variable dimensions. It is in the throat that a high speed fuel delivery orifice is usually located. The surface of this fuel delivery orifice is usually parallel to the air flow, or in other words perpendicular to a radius of the bore. Locating the fuel delivery orifice in the throat perpendicular to a bore radius gives the maximum fuel flow possible for a given orifice, bore design, and engine operating condition.
The fuel head pressure is simply the pressure required to raise the fuel against gravity a height equivalent to the difference between the level of the fuel delivery orifice in the bore and the level of the fuel in the fuel chamber. It is important that this level be controlled for uniform operation of the carburetor.
Many circuits, bleed systems, accelerator pumps, and other contrivances have been developed over years of carburetor use which modify to some extent this basic operating principle to shape the fuel delivery flow curve as a function of engine operating conditions, but in all carburetors this is the basic underlying operation. As can be seen by the above discussion, to assure the desired operation of the carburetor, the level of fuel in the carburetor, the pressure on the fuel, the size of the fuel metering orifices, and the size and shape of the bore, must be designed in accordance with overall operating parameters.
There are two basic types of carburetors, float bowl carburetors and wet diaphragm carburetors. In a typical float bowl type carburetor, fuel flows from a larger fuel tank into the float bowl of the carburetor, the level of fuel in the float bowl being determined by a float-actuated valve. This system is well known in the prior art and is discussed in my co-pending application number 08/664,187 filed on Jun. 14, 1996 now U.S. Pat. No. 5,772,928. This fuel level, as discussed above, is important in determining overall carburetor performance. In this case, the venting system used, whether internal, external, or a combination of both, determines the pressure existing in the air occupying the space above the fuel internal to the carburetor. This pressure may contain pressure pulses due to fuel inlet valve instability or due to pressure pulses in the carburetor bore, but the average of this pressure is one parameter which determines average carburetor fuel delivery.
In a typical wet diaphragm type carburetor, fuel flows under pressure from the larger fuel tank to the carburetor, and the pressure internal to the carburetor is controlled by a diaphragm-operated valve. This is also discussed in Ser. No. 08/664,187. In this case, there is no fuel level specifically, as there is no void internal to the carburetor; it is completely filled with fuel. In this type carburetor, the dry side of the diaphragm, or the side of the diaphragm opposite the side in contact with the fuel, is housed in a chamber which is either internally or externally vented. The average pressure of this chamber, while not being the actual pressure existing on the fuel internal to the carburetor, is the average reference pressure which determines the fuel pressure internal to the carburetor. This average reference pressure exists on the dry side of the diaphragm, while the fuel pressure internal to the carburetor exists on the wet side of the diaphragm. The movement of this diaphragm positions the moveable member of an inlet valve, and hence regulates the average fuel pressure in the carburetor and therefore helps determine average carburetor fuel delivery.
Internal venting can cause unwanted increases in fuel delivery at certain engine speeds. This is believed to be caused by pressure pulsations in the inlet manifold to the carburetor as discussed in U.S. Pat. Nos. 3,814,392 to Boyd et al (1974), and 5,273,008 to Ditter (1993). U.S. Pat. No. 3,814,392 to Boyd discusses the use of a winding maze in the internal vent passage. This winding maze is claimed to reduce the erratic nature of fuel flow at a critical engine speed. U.S. Pat. No. 5,273,008 to Ditter discusses the use of a primary internal vent with a small external vent to bleed off some of the vacuum. This is claimed to improve the fuel flow consistency by effectively "decoupling" or isolating the carburetor reference pressure from the induction tract pressure pulsations.
Internal venting is also claimed to be a cure for erratic fuel delivery caused by pressure pulsations in the carburetor bore by closely coupling the carburetor reference pressure to these bore pressure pulsations. U.S. Pat. No. 5,133,905 to Woody et al (1992) describes how a fixed Pitot tube (defined as a pressure sensing tube with an open end facing directly into a moving stream of gas) can be used in an internal vent system to reduce pulsations in the fuel delivery by balancing an instantaneous change in reference pressure against the bore pulsation. It is even described how a fixed predetermined spatial relationship between the location of the Pitot tube and the outlet of the fuel delivery system can be used to modify the phase relationship of the carburetor reference pressure to the bore pressure waves at the location of the fuel outlet. The bore pressure sensing device is always described as a Pitot tube, hence this device is a total pressure sensing device. As such, it essentially has no ability to change the magnitude of the average carburetor reference pressure with subsequent modification of average fuel flow. Also, the invention has no adjustment mechanism which could be used to tune the carburetor for changes in atmospheric conditions or engine operating parameters.
Prior art has discussed the use of internal vents to regulate the flow of fuel to an engine. U.S. Pat. Nos. 1,799,585 to Ensign (1931), 1,785,681 to Goudard (1926), 1,740,917 to Beck (1926), and U.S. Pat. No. 1,851,711 to Linga (1932) use an internal vent orifice in the carburetor bore which has its pressure affected by the throttle position. All of these devices are complex and add considerably to the cost of machining the carburetor. Except for U.S. Pat. No. 1,740,917 to Beck, they are not externally adjustable, and consequently are not usable to change fuel flow necessitated by a change in atmospheric conditions, for instance. They only change the shape of the fuel flow versus engine demand curve, and for any fixed set of operating conditions, the fuel flow is determined and not adjustable. U.S. Pat. No. 1,740,917 to Beck uses an internal vent positioned adjacent the throttle, and uses an adjustable (throttled) external vent to provide an external adjustment of the fuel flow.
Changes in atmospheric conditions, such as temperature, relative humidity, barometric pressure, and elevation, all of which determine the relative air density, have a considerable affect on the fuel delivery requirements of the engine. Changes in relative air density, without a corresponding change in carburetor tuning, result in engine loss of power or a waste of fuel. For instance, a snowmobile engine, which has carburetors tuned or jetted for proper operation at -20 degree Fahrenheit, will run overly rich and get the "blubbers" when run at +40 degrees Fahrenheit, unless the carburetor has the main jets changed to lean the mixture. In extreme cases, the mid-range operation of the carburetor must also be modified by moving or replacing a needle which affects the effective size of the fuel delivery orifice at part-throttle positions. The main jets, of necessity, are located near the bottom of the float bowl, and the changing of these jets is time consuming and results in loss of fuel. Repositioning or changing the mid-range needles is also time consuming and requires partial disassembly of the carburetor. Some carburetors have external mixture screws which adjust an opening in a fuel feed port which parallels the main jet. These systems are expensive to manufacture, and each carburetor must have its own adjustment system. Also these systems work by changing the effective size of the fuel delivery orifice, and not by changing the pressure acting to move the fuel through the fuel delivery system.
U.S. Pat. No. 5,021,198 to Bostelman (1990) describes a carburetor altitude compensation system using a combination of vents to regulate the fuel flow through the carburetors. In this system, an aneroid bellows is used to position a valve (choke), which changes the relative pressure effect of two orifices. One orifice is in the venturi region of the carburetor bore, and hence provides a vacuum to the float bowl which tends to decrease the flow of fuel. The other orifice is connected to a region of essentially atmospheric pressure, for instance the air cleaner, and tends to establish the float bowl pressure at atmospheric pressure, at which maximum fuel flow will occur. The valve is located in the high (atmospheric) pressure vent line and it "throttles" the flow of air through the high pressure line. As the valve moves toward the closed position, the increased throttling reduces the effect of the high pressure line relative to the low pressure line, lowering the pressure in the vent system. The consequent reduction in float bowl pressure results in a reduction of fuel flow through the carburetor. This system, like the adjustment in U.S. Pat. No. 1,740,917 to Beck, establishes a pressure by balancing two series orifices, one orifice being variably throttled. This is a tricky business considering the low pressure differentials existing in the carburetor system. Also, a throttling operation, such as used in these systems, is thermodynamically different than the essentially isentropic gas flow existing in a venturi. It is desirable to change fuel flow as uniformly as possible over the largest range of engine operating conditions. This uniformity is more readily accomplished using similar thermodynamic principles in the fuel delivery system and in the regulating system. Systems operating on a similar principle using valves and thermodynamic throttling are described in U.S. Pat. Nos. 4,660,525 to Mesman (1987), 4,574,755 to Sato et al (1986), 4,376,738 to Reinmuth (1983), 3,968,189 to Bier (1976), 3,789,812 to Berry et al (1974), and 3,730,157 to Gerhold (1973). All of these systems use a valve of some nature, usually of a needle type or a sliding type, which is usually operably closed to create an effective valve opening small compared to the area of the conduits to which it is attached, and perform a throttling operation to reduce the total pressure after passing the valve.
It is important to note that the fuel regulation systems using a valve in conjunction with a fixed orifice, or a valve in conjunction with another valve, use the throttling effect of the valve(s) to establish an intermediate pressure between two different pressure sources. A throttling operation is thermodynamically defined as an operation in which entropy is not constant; a throttling operation is not isentropic. Another property of a throttling operation is that total pressure is not constant, but is lower after the throttling has occurred. This reduction in total pressure is then used to modify the average reference pressure of the carburetor, and hence modify fuel flow.
Carburetors also have various components such as the above mentioned needles moving with the throttle valve to change the effective size of the fuel delivery orifice, pilot circuits to provide a transition from the low speed circuit to the high speed circuit, accelerator pumps to provide additional fuel during rapid acceleration, power jet circuits to provide additional fuel under full power operation, and variable venturis. All of these components and systems provide shaping of the fuel delivery as a function of engine operating conditions. These systems again are expensive to manufacture, and in many cases, are not externally adjustable, but require carburetor disassembly for their adjustment.